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Primary hypertension is considered a polygenic, inherited disorder; to date, the nature of the genes involved remains unknown. In this study we present evidence for a structural difference in the gene coding for renin between the stroke-prone, spontaneously hypertensive rat (SHRSP) and its normotensive control, the Wistar-Kyoto rat (WKY). Restriction fragment analysis using hybridization against probes complementary to defined regions of the renin gene identified a deletion, approximately 700 base pair in size, within the first intron in SHRSP compared with WKY. This restriction fragment length polymorphism (RFLP) affects a part of the gene that is characterized by the presence of a multimeric tandem repeat element, where the occurrence of insertional/deletional events might be expected and have recently been shown in other rat strains. In order to test for a possible phenotypical representation of this RFLP, we studied a population (n=115) of F2 hybrid rats derived from cross-breeding SHRSP with WKY. Using direct blood pressure measurements in conscious animals, we ruled out a cosegregation of systolic or diastolic blood pressure with renin genotype. Several other phenotypical parameters examined (heart rate, absolute and relative magnitude of changes in blood pressure induced by stress or dietary sodium, loading, plasma renin activity, ventricular hypertrophy and tissue water content) also showed no cosegregation with genotype. Our findings are in contrast to a recently published study examining an RFLP of the renin gene distinguishing salt-sensitive and salt-resistant Dahl rats. Thus, cosegregation of genotype and phenotype are not consistent, although in both cases, structural differences in the same region of the renin gene separate the hypertensive strain from its normotensive controls. These data may suggest differential roles of the renin—angiotensin system in these two models of genetically predetermined hypertension.